Morphologic and biochemical studies were made of cephalic vein bypass grafts that were interposed in the femoral artery in stumptailed macaque monkeys. These animals were maintained on a diet that sustained plasma cholesterol levels of 225 mg/dl. Comparisons were made of findings in control animals and in animals treated with dipyridamole and aspirin for as long as 18 months. Results obtained showed that cholesterol content of implanted grafts increased considerably compared to that of unimplanted veins, and that such an increase was much less pronounced in animals treated with dipyridamole and aspirin than in untreated animals; however, intimal thickening and medial fibrosis in vein grafts was not affected by treatment with antiplatelet agents.